The silent period observed in electromyographic recordings from the jaw closing muscles in response to a tap on the chin provide a quantitative, objective, physiological probe for temporomandibular joint (TMJ) syndrome: patients with TMJ syndrome exhibit prolonged silent periods when compared to normal subjects. This syndrome involving pain and dysfunction of the TMJ and muscles of mastication is widespread in the general population, has controversial etiological theories, and has equally controversial treatment modalities. This study will investigate both the clinical utility of, and the neuromuscular mechanisms underlying these silent periods. We will investigate the latency and duration of the silent period on the clinical level by elucidating the role of the mechanical input parameters on the silent period, by testing the specificity of the prolonged silent period to TMJ patients in comparison with other patients reporting orofacial pain, and by comparing statistically the silent period latency and duration with a clinician's estimate of symptom severity. The neural mechanisms underlying the silent period will be studied to assess the relative contributions of receptors in the periodontal ligament, the primary endings of the muscle spindles, and cutaneous afferents to the latency and duration of the silent period. The clinical experiments will show under what conditions the silent period may be used as a quantitative measure for differential diagnosis. The physiological experiments will reveal the source of inhibition observed in the TMJ patients as well as provide additional knowledge about the trigeminal neuromuscular system.